The temperature-sound pressure characteristic is extremely important in ultrasonic transducers used in ultrasonic sensors and so forth. This is because even if a sufficient output sound pressure is provided at a certain temperature, the output sound pressure may decrease when the temperature changes and an ultrasonic wave may no longer arrive at the target and as a result the applications in which such ultrasonic transducers can be used is limited. For example, it has not been possible to use an ultrasonic transducer having a poor temperature-sound pressure characteristic in a car-mounted ultrasonic sensor for distance measurement that has to be able to cope with a severe temperature environment.
In FIG. 16, an ultrasonic transducer 400 of the related art is illustrated that is disclosed in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2001-258098) and that has an improved temperature-sound pressure characteristic. The ultrasonic transducer 400 has a structure in which a piezoelectric vibrator 103 is affixed to a rear surface side of a metal plate 101 with adhesive 102 therebetween and in which conical funnel-shaped resonator (horn) 104 is attached to the front surface side of the metal plate 101. After providing cushioning material 105 on the rear surface side of the piezoelectric vibrator 103 the entire device is accommodated inside a case composed of a base member 106 and a cover 107.
In the ultrasonic transducer 400, in order to achieve an improvement in the temperature-sound pressure characteristic, the coefficient of linear expansion of the adhesive 102 is made to be larger than the coefficient of linear expansion of the piezoelectric vibrator 103 and the coefficient of linear expansion of the metal plate 101 is made to be smaller than the coefficient of linear expansion of the piezoelectric vibrator 103. In the ultrasonic transducer 400, expansion of the adhesive 102 that occurs with a change in temperature is suppressed by the metal plate 101 and as a result stress that would cause deformation that would lead to warping of the piezoelectric vibrator 103 is reduced and therefore the size of a change in frequency caused by a change in temperature is reduced and the temperature-sound pressure characteristic is improved.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-258098
In the above-described ultrasonic transducer 400 of the related art, the coefficients of linear expansion of the piezoelectric vibrator 103, the metal plate 101 and the adhesive 102 are adjusted with respect to each other, the adhesive 102 adhering the piezoelectric vibrator 103 and the metal plate 101 to each other, and as a result stress acting on the piezoelectric vibrator 103 is reduced and it is considered that there is some effect of improving the temperature-sound pressure characteristic. However, in the ultrasonic transducer 400, since measures have not been taken with respect to temperature characteristics of the resonant frequency, mechanical quality coefficient (Qm), electrostatic capacity, piezoelectric constant (d constant) and so forth of the piezoelectric vibrator 103 itself, significant improvements in the temperature-sound pressure characteristic have not been achieved.